Please give some more details about the situation.
1. Do you receive any bytes of data?
Does Serial.println(Marvelmind.available()) print anything except zero?
2. Have you frozen the submap?
Normal streaming mode of mobile beacon is when the map is built and frozen, and the mobile beacon updates its position.
Please share screenshot of map in dashboard when modem is connected.
3. Do you try to get the data when mobile beacon is connected to dashboard?
When mobile beacon is connected to dashboard, it may be not stream data to output. Try to disconnect it from PC.
4. Please share photo of wiring connection of mobile beacon to Arduino.
Maybe your type of beacons doesn't contain compass, so you don't get compass data.
Only HW4.9 beacons with IMU and mini-Tx beacons have the magnetometer.
We don't recommend using compass indoor, because it is affected by any metal things which usually present indoor.
For getting heading we recommend use paired beacons instead the magnetometer, this gives more precise angle.